U.S. Pat. No. 4,145,847 issued Mar. 27, 1979 to Alma A. Hutchins on "Straight Line Abrading Tool With Balancing Counterweight" discloses a hand held abrading tool including a piston structure mounted within a cylinder body for reciprocation by air or other pressure fluid delivered to the cylinder chamber by a valve mechanism operating in timed relation to the piston structure. The piston structure drives a reciprocating shoe which carries a sheet of sandpaper or other abrading material, and also drives a counterweight preferably located at the underside of the body and above the shoe. The shoe reciprocates parallel to but oppositely from the piston structure, and the counterweight reciprocates essentially in unison with the piston structure. U.S. Pat. No. 5,001,869 issued Mar. 26, 1991 to Donald H. Hutchins on "Reciprocating Abrading or Polishing Tool With Balancing Counterweights"shows a similar straight line tool having additional counterweights at a level above the shoe and preferably at opposite sides of the piston structure.
In the tools of both of these patents, it is possible, when the delivery of fluid to the tool is shut off, for the moving parts and valve mechanism to stop in a position from which the valve mechanism can not restart the tool when the pressure fluid is again turned on. In using the tools of the prior patents, it is then necessary to manually move the shoe relative to the body to a different position in which the valve mechanism can deliver fluid to the piston structure in a manner recommencing the reciprocating motion.